1. Field of the Invention
An aspect of the present invention relates to an image display apparatus, and, more particularly, to an image processing apparatus and a method of reducing power consumption of a self-luminous display.
2. Description of the Related Art
Recently, display apparatuses have been introduced in response to the development of computers and the spread of the Internet. These display apparatuses are embedded in a wide variety of devices ranging from devices that require relatively large displays (such as digital televisions (TVs) and monitors), and to portable devices that require small and convenient displays (such as cellular phones and personal data assistants (PDAs)). Unlike the large devices, portable devices are powered by charging type batteries. Therefore, reducing power consumption of the portable devices to increase the time during which the portable devices can be used is important.
Display apparatuses are largely classified into transmissive display apparatuses (such as liquid crystal displays (LCDs)), and self-luminous display apparatuses (such as plasma display panels (PDPs), and organic light emitting diodes (OLEDs)).
FIG. 1 illustrates the light-emitting principle of a conventional LCD 10. The LCD 10 receives a white backlight 11 from a backlight unit and either passes the white backlight 11 through a liquid crystal layer 12 or blocks the white backlight 11. The transmittance of the backlight 11 is controlled by varying the arrangement of electrodes 13 formed on both surfaces of the liquid crystal layer 12 according to a voltage applied to the electrodes 13. Here, the transmitted light is converted by a color filter 14 into a color 15 and then output to the exterior of the LCD 10. To reduce power consumption, transmissive display apparatuses, such as the LCD 10, use a method of uniformly adjusting the brightness of a backlight source regardless of image information because the power consumed by the backlight source remains unchanged regardless of whether the image information indicates black or white regions.
A conventional technology for reducing the power consumption of a transmissive display apparatus has been disclosed by Samsung Electronics Co., Ltd. in Korean Patent Publication No. 2005-0061797. Here, a driving voltage level is controlled using an average luminance value received. Hence, when the average luminance value is greater than a predetermined value, the amount of light is reduced, and when the average luminance value is less than the predetermined value, the amount of light is increased. In so doing, power consumption of the transmissive display apparatus may be reduced while the deterioration of the overall luminance of the transmissive display apparatus may be prevented. In addition, Toshiba Corporation discloses, in Japanese Patent Publication No. 2004-246099, another conventional technology for extracting a luminance signal component of an input signal, highlighting the extracted luminance signal component, and then reducing the amount of light of a backlight.
FIG. 2 illustrates the light-emitting principle of a conventional OLED 20. As shown in FIG. 2, electrodes 22 and 24 are formed on both surfaces of an organic thin film 23 of the OLED 20. Electrons are injected through these electrodes 22 and 24, and excitation of holes is formed. Light 26, having a particular wavelength, is generated by energy from the formed excitation. The conventional OLED 20 emits red, green and blue (RGB) colors according to the type of organic matter contained in the organic thin film 23, thereby representing a full color band. The intensity of the generated light 26 is determined by the intensity of current supplied from a power source 21.
A conventional technology to reduce power consumption of a self-luminous display apparatus has been disclosed by Samsung SDI Co., Ltd. in Korean Patent Publication No. 2004-0069583. Specifically, this conventional technology relates to a plasma display calculating an average luminance level of an input image, and, if the average luminance level is less than a predetermined level, calculating the difference between average luminance levels of frames and then reducing the power consumption of a current frame. In addition, Korean Patent Publication No. 2004-0070948 assigned to Samsung Electronics Co., Ltd. discloses a technology to calculate an average luminance level of an input image, to set a power consumption level, and to display the input image on a PDP according to the set power consumption level. Also, U.S. Patent Publication No. 2006-0044227 assigned to Kodak discloses a technology for generating a calibration curve indicating the relationship between a driving voltage and current (luminance) in an OLED and controlling the driving voltage based on the calibration curve.
Low-power technology can be used to reduce the power consumption of transmissive display apparatuses. However, since self-luminous display apparatuses inherently do not have backlights, the efficiency of power consumption of the self-luminous display apparatuses can be enhanced only by reducing the size of an input signal. In other words, while transmissive display apparatuses consume a constant level of power regardless of luminance, the luminance of self-luminous display apparatuses is proportional to an amount of flowing current (power consumption).
FIG. 3 illustrates power consumed by a self-luminous display apparatus according to characteristics of an image displayed thereon. Theoretically, when a black image is displayed on the self-luminous display apparatus, the power consumption of the self-luminous display apparatus is nearly 0%. When a white image is displayed, the power consumption of the self-luminous display apparatus is nearly 100%. In the case of a general image, the power consumption is somewhere between 0 and 100%.
A still image consumes 50-60% of total power, whereas a moving image consumes relatively less power, i.e., 20-30% of the total power. In addition, a black character in a white background consumes more power (70-80% of the total power) than a white character in a black background (20-30% of the total power).
As is described above, since self-luminous display apparatuses control brightness using the amount of current, they consume a lot of power when emitting bright light. Therefore, a reduction in power consumption is essential for the self-luminous display apparatuses to be used for mobile devices to which it is difficult to supply power in a stable manner.
Most conventional technologies to drive LCDs and PDPs use a method of lowering backlight to a constant level by reducing voltage or displaying an input image at a power level set by flowing current according to a predetermined power consumption level. The above discussed OLED low-power technology disclosed by Kodak is also a voltage control method according to a predetermined power level.
However, if driving voltages for all signals of an image are uniformly lowered, the brightness of undesired portions of the image by a user is also lowered, thereby deteriorating image quality. Therefore, a technology to reduce power consumption by analyzing characteristics of an input image based on a human visual system and dynamically controlling a level of a signal (pixel value) based on the analyzed characteristics of the input image is required.